1. Field of the Invention
The present invention relates to a method and an apparatus for transforming the physical characteristics of a material and particularly a method and apparatus for molding by vibration (either mechanical or electrical) to enable the control or modification of the physical properties of the molded materials, notably their mechanical and optical properties, as well as the novel materials so obtained.
2. Description of the Prior Art
Modern improvements in classical molding technologies include the control of certain fabrication variables during, or subsequent to shaping, in order to increase the end-use performance of the finished product. In such diverse operations as rotational molding, blow molding or thermoforming, shaping is applied to an already preformed material. This technique permits the incorporaof means to impart improved characteristics in the processing method by structuring the material morphologically.
Probably one of the most significant factors influencing the final end-use performance of a material is the rate at which it has been cooled through its thermokinetic transition temperatures, that is, its melting temperature or its glass transition temperature. For crystalline and semi-crystalline materials, the number, size, type and distribution of crystallites produced under specific cooling conditions dictate to a large extent the degree of crystalinity, density of the defects, if any, the importance of the amorphous regions as well as the overall morphology which determine the performance of the finished product. It is well known, for instance, that crystallites act as fillers or physical crosslinks in a polymeric system, and as such they play an important role in the determination of the shear modulus of the sodified product. Also it is known that the degree of crystallinity determines the optical aspect of the end-use article, controlling its transparency to light. Complete transparency is generally achieved under certain circumstances for crystallizable material under cooling conditions which avoid crystallization.
Therefore, one sees that the physical and physicochemical characteristics of a material depend on the speed or the variations in speed of cooling of the material during its passage between a molten or pasty state and a solid state.
Amorphous non-crystallizable materials are materials able to condense into a glass, which is structurally similar to, but kinetically indistinguishable from, liquids. FIG. 1 depicts the volume changes as a glass-forming rubber is cooled through its glass transition temperature T.sub.g. All glass forming materials undergo a drastic property change on passing through this region, the precise value of which depends on the rate of cooling and other experimental factors, e.g., pressure and time. The state of a glassy system is a nonequilibrium one, reflecting a frozen-in disordered condition with strongly restricted molecular mobility. It is seen in FIG. 1 that an amorphous material cooled rapidly through its T.sub.g condenses at a higher temperature and that the specific volume of the quenched glass is greater than that of the slowly cooled material. The physical properties of amorphous thermoplastics in the glassy state can vary considerably with the rate of cooling through T.sub.g (alternatively with subsequent annealing treatment below T.sub.g), as is well known for the impact resistance of polycarbonate. Impact strength is the ability to dissipate the energy of an impact through some mechanical loss process without breaking of the material. The reason why unmodified amorphous polymers display such a large variety of impact behaviors, from the tough polycarbonate to the brittle polystyrene, is not well understood. Of all the unmodified amorphous polymers, polycarbonate has one of the highest values for impact strength. Yet, it has been reported that polycarbonate can completely lose its impact energy when it is extensibly annealed below its glass transition temperature. This same drastic loss of the impact characteristics of polycarbonate would be observed by cooling it slowly through its T.sub.g to permit relaxation under non-isothermal cooling conditions.
The existence of residual stresses in thermoplastics, due to forming assembly processes and to post-treatments, is a well known problem to design and applications engineers. Residual stresses can be enhanced by quenching and released by annealing below T.sub.g. Once the residual stresses are formed, then the rate at which they relax is controlled by the ability of the material to recover to its equilibrium state. The residual stresses influence the optical and mechanical behavior of the engineered product since they contribute to the total stress level, therefore a knowledge of residual stresses, and means to control its level, are important when selecting the maximum allowable external stresses which can be applied to the thermoplastic article.
Another important influence on the rate of cooling is illustrated in the formation of compatible blends of materials. A polyblend is a blend of two or more substances. For instance, for macromolecular substances, a polyblend would be a mixture of chemically different powders, resins or elastomers. From both thermodynamic considerations and experimental observations, when polymers of different compositions are mixed they do not intermix down to the molecular level and therefore do not provide a homogeneous single-phase structure. The ultimate state of molecular mixing attainable by molecular mixtures can only be approached by polymeric polyblends up to a limit. Conceptually, compatability can be a representation of how close the polyblend approaches the ultimate state of molecular mixing as a limit. Hence, compatability can best be described by the degree of homogenity of the polyblend and measured and compared by the domain size of the dispersed phase. The thinner the size of the dispersed phase in the continuous phase, the better the compatability of the polyblend.
The compatability of a molten blend of two or more polymers is a function of temperature. The higher the temperature, the better the compatability. The perfection of compatability and its dependence on temperature and time are rate or diffusion sensitive processes--it is a function of the cooling rate imposed on the molten blend in the mold cavity while being cooled to room temperature.
A supercooled blend or a blend quenched at infinite rate would theoretically have the compatability characteristics it had at the temperature from which the quench started. Thus this is another important consideration on the effect of the cooling rate on the final characteristics of the finished product.
Thermal history during forming plays an important role in the determination of the properties of the end finished product. Therefore the ability to monitor the rate of temperature variation while the material is processed, in order to alter its properties, is a major technological objective.
Studies, carried out up to the present, in the laboratory, had consisted of working at constant hydrostatic pressure and modifying the cooling speed of the material by insulating the material or by accelerating its heat exchange to the outside.
Known methods consisting of modifying the heat exchange between the material in the course of cooling and the outside, are limited by the restrictions of heat transfer (conduction, convection, or radiation). Accordingly, it is not always possible to modify to a great extent the speed of heat exchange or the variation in temperature within sufficient limits to influence the physical characteristics of the end-finished molded product once it had reached ambient temperature.
The classic approach in metallurgy to the problem of obtaining non-equilibrium phases has been rapid removal of heat by convection. Generally, a molten metal is dropped into a cooling fluid, such as liquid nitrogen. With specimens, a few microns thick, cooling rates on the order of ten thousand or even twenty thousand degrees centigrade per second have been attained. Due to the poor heat conductivity of certain non-metallic materials, such as polymeric materials, special attention must be paid to simple size and geometry considerations in quenching experiments. While the extremely high cooling rates employed in metallurgy are not necessary in preparing completely amorphous polymeric samples, the technique can only be used on very thin samples and therefore industrial applications are indeed very limited.
Numerous materials are transformed today from an initial crude state to a finished or semi-finished article by molding operations. These materials are, for example, metals, glasses, ceramics, polymers, resins and rubbers; organic or inorganic; synthetic or natural. These materials can also be blends of several materials.
These materials are each endowed with a characteristic set of mechanical and physical properties which depend on its chemical nature, but which also strongly depend on its thermodynamic state after molding, in particular, the state of thermodynamic nonequilibrium at the temperature of use, which to a large extent depends on its thermal history during molding.
Various researchers have sought to modify and improve certain physical and mechanical characteristics of materials by controlling the parameters of the molding process. For instance, the influence of a constant hydrostatic molding pressure on the mechanical and relaxation properties of both amorphous and semi-crystalline polymeric materials have been the subject of extensive investigations.
It is known that a constant hydrostatic pressure applied during cooling modifies the change of state of materials, for example, in the vitrification process. This influence is manifested by the fact that the temperature of vitreous transition (T.sub.g) is a function of the constant hydrostatic pressure exerted on the liquid material during cooling as shown in FIG. 2. The value of T.sub.g reflects the thermokinetic instability state of the vitrified material or glass. This signifies that it is theoretically possible to condense this material to the vitreous state at any temperature, on condition that a sufficient constant hydrostatic pressure is applied to raise the vitreous transition temperature (T.sub.g) at least up to the temperature concerned. These phenomena have been described experimentally in the literature. See G. Allen et al., J. Polymer. Sci., C, 23, 127 (1968), W. C. Dale et al., J. Appl. Polymer. Sci., 16, 21 (1972), and E. Jones Parry and D. Tabor, J. Materials Sci. 8, 1510 (1973).
However, it is not suggested anywhere in these publications to vary the hydrostatic pressure during the cooling of the material in a manner which is controlled by the variation of the temperature of the material itself for the purpose of controlling or even "tailoring to order" the properties of the final end-use product.
The Bogulavsky et al. patent (U.S. Pat. No. 3,912,480) describes a method for annealing glass by bringing it to its annealing temperature (below T.sub.g), maintaining it at this temperature in a fused salt bath, and subjecting it simultaneously to mechanical vibration of fixed frequency between 20,000-18,000 hertz, transmitted by the fused salt (whose viscosity does not exceed 100 hertz) to the glass. This method is not intended to cause a change in state and operates at a fixed frequency and temperature contrary to the present invention. Furthermore, it is only applied to mineral glass, without mentioning other materials.
U.S. Pat. No. 4,150,079 to Chang involves a method for controlling (suppressing or enhancing) crystallization in crystallizable thermoplastic polymeric materials by varying hydrostatic pressure during cooling. It is not suggested in Chang to apply vibrational means (either mechanical or electrical) superimposed on the action of the hydrostatic pressure to determine specific cooling patterns.